While their attention may be inland on the San Andreas Fault, residents
of coastal Southern California could be surprised by very large
earthquakes -- and even tsunamis -- from several major faults that lie
offshore, a new study finds.
The latest research into the little known, fault-riddled, undersea
landscape off of Southern California and northern Baja California has
revealed more worrisome details about a tectonic train wreck in the
Earth's crust with the potential for magnitude 7.9 to 8.0 earthquakes.
The new study supports the likelihood that these vertical fault zones
have displaced the seafloor in the past, which means they could send out
tsunami-generating pulses towards the nearby coastal mega-city of Los
Angeles and neighboring San Diego.
"We're dealing with continental collision," said geologist Mark Legg
of Legg Geophysical in Huntington Beach, California, regarding the cause
of the offshore danger. "That's fundamental. That's why we have this
mess of a complicated logjam."
Legg is the lead author of the new analysis accepted for publication in the Journal of Geophysical Research: Earth Surface,
a journal of the American Geophysical Union. He is also one of a
handful of geologists who have been trying for decades to piece together
the complicated picture of what lies beyond Southern California's
famous beaches.
The logjam Legg referred to is composed of blocks of the Earth's
crust caught in the ongoing tectonic battle between the North American
tectonic plate and the Pacific plate. The blocks are wedged together all
the way from the San Andreas Fault on the east, to the edge of the
continental shelf on the west, from 150 to 200 kilometers (90 to 125
miles) offshore. These chunks of crust get squeezed and rotated as the
Pacific plate slides northwest, away from California, relative to the
North American plate. The mostly underwater part of this region is
called the California Continental Borderland, and includes the Channel
Islands.
By combining older seafloor data and digital seismic data from
earthquakes along with 4,500 kilometers (2,796 miles) of new seafloor
depth measurements, or bathymetry, collected in 2010, Legg and his
colleagues were able to take a closer look at the structure of two of
the larger seafloor faults in the Borderland: the Santa Cruz-Catalina
Ridge Fault and the Ferrelo Fault. What they were searching for are
signs, like those seen along the San Andreas, that indicate how much the
faults have slipped over time and whether some of that slippage caused
some of the seafloor to thrust upwards.
What they found along the Santa Cruz-Catalina Ridge Fault are ridges,
valleys and other clear signs that the fragmented, blocky crust has
been lifted upward, while also slipping sideways like the plates along
the San Andreas Fault do. Further out to sea, the Ferrelo Fault zone
showed thrust faulting -- which is an upwards movement of one side of
the fault. The vertical movement means that blocks of crust are being
compressed as well as sliding horizontally relative to each other-what
Legg describes as "transpression."
Compression comes from the blocks of the Borderland being dragged
northwest, but then slamming into the roots of the Transverse Ranges --
which are east-west running mountains north and west of Los Angeles. In
fact, the logjam has helped build the Transverse Ranges, Legg explained.
"The Transverse Ranges rose quickly, like a mini Himalaya," Legg said.
The real Himalaya arose from a tectonic-plate collision in which the
crumpled crust on both sides piled up into fast-growing, steep mountains
rather than getting pushed down into Earth's mantle as happens at some
plate boundaries.
As Southern California's pile-up continues, the plate movements that
build up seismic stress on the San Andreas are also putting stress on
the long Santa Cruz-Catalina Ridge and Ferrelo Faults. And there is no
reason to believe that those faults and others in the Borderlands can't
rupture in the same manner as the San Andreas, said Legg.
"Such large faults could even have the potential of a magnitude 8
quake," said geologist Christopher Sorlien of the University of
California at Santa Barbara, who is not a co-author on the new paper.
"This continental shelf off California is not like other continental shelves -- like in the Eastern U.S.," said Sorlien.
Whereas most continental shelves are about twice as wide and
inactive, like that off the U.S. Atlantic coast, the California
continental shelf is very narrow and is dominated by active faults and
tectonics. In fact, it's unlike most continental shelves in the world,
he said. It's also one of the least well mapped and understood. "It's
essentially terra incognita."
"This is one of the only parts of the continental shelf of the 48
contiguous states that didn't have complete ... high-resolution
bathymetry years ago," Sorlien said.
And that's why getting a better handle on the hazards posed by the
Borderland's undersea faults has been long in coming and slow to catch
on, even among earth scientists, he said.
NOAA was working on complete high-resolution bathymetry of the U.S.
Exclusive Economic Zone -- the waters within 200 miles of shore -- until
the budget was cut, said Legg. That left out Southern California and
left researchers like himself using whatever bits and pieces of smaller
surveys to assemble a picture of what's going on in the Borderland, he
explained.
"We've got high resolution maps of the surface of Mars," Legg said,
"yet we still don't have decent bathymetry for our own backyard."
This story is taken from Science Daily
Historic satellite observations and future climate model projections
were used to estimate past and future changes in habitat suitability
during the penguins' chick-rearing period.
